Typhoid fever, caused by the intracellular bacterium Salmonella typhi, is responsible for more than a half million deaths annually worldwide. In a number of infected people, Salmonella bacteria persist in the face of a potent adaptive immune response, generally maintaining reservoirs within specific organs such as the liver, spleen, and mesenteric lymph nodes. While it is known that IFN-? and CD4 T cells are required to control Salmonella infection, the immune mechanisms that regulate bacterial persistence remain unclear. Here, we provide evidence that endogenous Salmonella-specific CD4 T cells from lymphoid tissue protect against new Salmonella infections, while those from the liver actually worsen infection. We also find important functional and phenotypic differences between lymphoid tissues and liver, which may contribute to bacterial persistence. The main goal of this project is to test our central hypothesis that persistent Salmonella infection is due to the suppressive nature of Salmonella-specific CD4 T cells in the liver compared to the effector functions of those in secondary lymphoid tissue. We hypothesize persistent Salmonella infection de- pends upon two factors: (1) new thymus-derived T cells entering the effector T cell pool and (2) differences in antigen presentation between lymphoid and hepatic tissue. This hypothesis is based on our strong preliminary data. Specifically, this project will 1) Define how Salmonella-specific CD4 T cells from secondary lymphoid tis- sues and liver affect Salmonella persistence in vivo; 2) Determine the contribution of new, thymus-derived CD4 T cells to Salmonella persistence in vivo; and 3) Determine how organ-specific antigen presentation affects the functional phenotype of Salmonella-specific CD4 T cells during persistent infection. The research proposed is innovative due to the combination of the ability to assess endogenous CD4 T cell response using specific MHC class II tetramers made in our lab combined with novel insights into how the anatomical location of bacteria impacts CD4+ T cell responses to persistent infection. This contribution is significant because it will provide new insights into immunological mechanisms that underlie the persistence of an intracellular bacterium in vivo, as well as define the role tissue microenvironments play in modulating the pathogen-specific immune response. Ultimately, this knowledge can serve as a springboard for understanding how tissue environment shapes the endogenous antigen-specific response to other persistent human bacterial pathogens.